A recent study has cast doubt on the theory that bacteria hasten death and that living in a sterile environment would make you live longer, reported the BBC on August 10 2007.

The report went on to say, "It has been thought that the immune system response provoked by even harmless bacteria speeds up the ageing process by using up vital energy."

This story is based on a study of fruit flies which showed similar survival between those bred in a bacteria-reduced environment and those who weren't ("their grubby siblings").

The researchers set out to look at whether the number of bacteria on fruit flies increases with age and whether presence of bacteria affects lifespan. A previous study had found that the presence of bacteria was beneficial for fly survival. The researchers made no attempt to extrapolate these findings to what might happen in humans.

Our assessment is that this is a study of flies, and the findings do not apply to humans. As the news story and the study's researchers appropriately point out, humans require certain bacteria for proper digestion and other functions. Without these, health in humans would be compromised. It is difficult therefore to see how these results affect our understanding of ageing in humans.

Where did the story come from?

Ren and colleagues from the University of Southern California and the House Ear Institute, both in Los Angeles, CA, conducted this study. The study was published in the peer-reviewed medical journal Cell Metabolism .

What kind of scientific study was this?

This study was a laboratory study conducted on fruit flies. Researchers compared the lifespan of fruit flies bred in a bacteria-limited environment with those bred in a normal environment.

What were the results of the study?

The following findings are most relevant to this story:

In flies living in a normal environment, the concentration of bacteria on their bodies increased as the flies aged; no bacteria were found on old flies bred in the bacteria-reduced environment.

Immune response was found to increase with age in those flies living in a normal environment (as expected because of increasing bacterial load). This did not happen in those flies living without bacteria.

There was no difference in lifespan between old flies with large number of bacteria on their bodies and those without the bacteria.

What interpretations did the researchers draw from these results?

The researchers conclude that “any metabolic expenditure by the fly required to support the bacterial load and the innate immune response occurs without a cost to life span”. The researchers do not attempt to extend their findings to human ageing.

What does the NHS Knowledge Service make of this study?

This was a study conducted on fruit flies and as a result, its findings are of little consequence to our understanding of the human lifespan. It is well established that certain bacteria are essential for digestion and other functions in humans; without them, human health would be compromised. There are likewise many disease causing bacteria that are very detrimental to health.

This study has limited relevance to the complex factors involved in health, disease and longevity in humans.